Pure titanium and titanium alloy are metal materials which are indispensable in chemical/electrical plants or in high-value-added products such as airplane, sports equipment, for having an excellent lightness, thermal resistance and corrosion resistance. Titanium metal products which are produced from such pure titanium and titanium alloy are manufactured through processes of rolling or forging to a titanium ingot. As a technique of producing a titanium ingot, there are Consumable Electrode Vacuum Arc Remelting VAR (Vacuum Arc Remelting) method, Hearth Melting EB (Electron Beam) method which uses electron beam, Hearth Melting PAM (Plasma Arc Melting) method which uses plasma arc, which will be explained below.
The Consumable Electrode Vacuum Arc Remelting VAR method is a technique which has been conventionally widely used as a method of melting a titanium ingot comprising pure titanium or a titanium alloy. The VAR method is a method in which an arc (DC arc) is generated in a melting furnace in an atmosphere of high vacuum or an inert gas (Ar, He) between a consumable electrode which is prepared in advance by using a raw material of titanium ingot and a molten metal in a water-cooled copper crucible, and the consumable electrode is melted by using the arc as a heat source, to thereby obtain a titanium ingot from the molten metal of the melted consumable electrode.
In the VAR method, in order to completely melt the raw material of the titanium ingot to homogenize chemical composition of the titanium ingot, usually, a second melting is performed by using the titanium ingot obtained in the first melting as a consumable electrode. In particular, in titanium alloys for aircraft use, the melting is sometimes performed for three times for further homogenization of chemical composition of titanium ingot to reduce segregation of chemical composition.
Hearth melting EB method is a technique of producing a titanium ingot by supplying raw materials comprising melted titanium sponge, scrap or the like to a water-cooled copper hearth, heating these raw materials by using electron beam as a heat source, pouring the heated material continuously into a water-cooled copper mold, and then continuously withdrawing the material from the mold. In this EB method, the withdrawal is performed with irradiating surface of the molten metal with electron beams in order to maintain uniformity of the molten metal temperature in the water-cooled copper mold and to suppress coagulation, in a high vacuum environment. In this time, by the irradiation with electron beams having a high energy density in a high vacuum environment, a metal with a low melting point such as Al having a high vapor pressure is evaporated, and therefore, it is difficult to control chemical composition of the materials. Therefore, it can be said that this EB method is a preferred technique mainly for production of pure titanium ingot.
Hearth melting PAM method is a technique for producing a titanium ingot by supplying raw materials comprising melted titanium sponge, scrap or the like to a water-cooled copper hearth, heating these raw materials by using plasma arc as a heat source, pouring the heated material continuously into a water-cooled copper mold, and then continuously withdrawing the material from the mold. In this PAM method, the withdrawal is performed with irradiating surface of the molten metal with an arc generated from a plasma torch in an inert gas environment. It can be said that PAM method is a preferred technique for production of ingot of titanium alloy, since it is carried out in an inert gas environment, the evaporation loss of the molten metal is relatively small, and the chemical composition control of the raw material is relatively easy.
Both the EB method and the PAM method are capable of producing a titanium ingot directly from raw materials, without need of preparing a consumable electrode as in the VAR method, and therefore, have attracted more attention as a melting method with higher productivity than that of the VAR method.
Patent Document 1 discloses a method for producing a metal ingot with a high melting point by performing withdrawing with irradiating surface of a molten metal with electron beam, which is an example of the EB method. The method for producing a metal ingot with a high melting point of Patent Document 1 is a method in which, while molten metal is supplied into a mold which constitutes an electron beam-melting furnace to form a mold pool, a cooled and solidified ingot part near the bottom of the mold pool is withdrawn with being turned to thereby produce a metal ingot with a high melting point, and in which the mold pool surface is irradiated such that energy density of the electron beams along the outer circumferential portion of the mold pool adjacent to the mold is enhanced relative to electron beams in the central portion of the mold pool among the electron beams with which the mold pool surface is irradiated.
As described above, the EB method employed in the technique of Patent Document 1 is a melting method of higher productivity than VAR method is, for being capable of producing a titanium ingot directly from raw material. However, due to use of electron beams, the method should to be carried out in a high vacuum environment, and therefore, is not suitable for producing ingot of titanium alloy which requires chemical composition control of the raw material.
Therefore in these days, hearth melting, in particular, a PAM method which has small evaporation loss is beginning to be recommended as a means of producing a titanium alloy ingot of homogeneous chemical composition with no internal defect. However, in the conventional PAM method, in producing an ingot of small segregation of chemical composition, there has been a limit in diameter of the ingot, and therefore, it has been difficult to suppress segregation of chemical composition in the titanium alloy to produce a high-quality ingot.
Specifically, in a casting method which uses the PAM method in which melted titanium alloy is poured into a mold and simultaneously the molten metal in the mold is downwardly withdrawn with being heated with plasma torch, heating the central portion of upper surface of the molten metal by plasma forms a molten metal pool in which the central portion is the most deep. The molten metal pool is a solidification interface position of molten metal. When diameter of a mold is increased in order to increase diameter of a titanium ingot to be withdrawn, the central portion of a molten metal pool becomes too deep, and segregation of chemical composition becomes noticeable.
It is said that limit of diameter for a titanium ingot to have an insignificant segregation of chemical composition is conventionally φ300 to 400 mm. As for a titanium alloy ingot, it is said to be φ900 mm (3 times melting) at maximum in the VAR method, and about φ500 mm at maximum in the PAM method. However, in order to obtain a product with an excellent mechanical characteristic such as fatigue strength by processing an ingot through a forging process and heat treatment to form a homogenous material construction, an ingot of a large diameter of φ800 mm or more, preferably, φ1,000 mm or more is required. Therefore, there has been desired a casting method capable of controlling segregation of chemical composition even in a titanium ingot and titanium alloy ingot with a large diameter to become equivalent to or less than a segregation of chemical composition in an ingot with a small diameter.